Question

There are a variety of places where I need to check if a JavaScript variable is null or empty string so I wrote an extension method that looks like this:

Object.prototype.IsNullOrEmptyString = function()
{
    return (this == null || (typeof this === "string" && this.length == 0));
}

I then call it like so:

var someVariable = null;

if (someVariable.IsNullOrEmptyString())
    alert("do something");

But that doesn’t work because, at the point of evaluation in the if statement, someVariable is null. The error I keep getting is “someVariable is null”.

You can see it live here: http://jsfiddle.net/AhnkF/ (run in Firefox and notice the error console)

Is there anyway to do what I want and have a single extension method check for null and other things at the same time?

Answer 1

null does not have any properties or methods. You have to create a function, and pass the variable, so it can be tested. Note: To test whether a variable is really an empty string, using === "" is recommended.^*

function isNullOrEmpty(test) {
    return test === null || test === "";
}

var someVariable = null;
if (isNullOrEmpty(someVariable)) alert("Do something");

^* about == and ===. The following comparisons are true:

null == undefined
"" == 0;
"" == false
"" ==